A monolayer and a multilayer film formed, for example, by the Langumuir Blodgett method (hereinafter referred to as "L-B method") or the like has the properties of an ultrathin film in which organic molecules are in a monodimensional orientation and are present in high density, and these are used in various fields according to their characteristic functions due to the properties.
Compounds which are suitable for formation of a monolayer are amphiphilic surfactant type molecules having both a hydrophilic group and a hydrophobic group. Such compounds are spread over the surface of an aqueous subphase from a volatile organic solution, and then pressure is applied thereto so as to compress the molecules under an appropriate surface pressure whereby the molecules are oriented such that the hydrophilic groups face downwards to the aqueous subphase surface and the hydrophobic groups face upwards from the surface and the molecules are densely packed so as to form a monolayer. Insoluble compounds wherein the whole molecule is hydrophobic cannot be oriented and therefore are easily aggregated. Such aggregation makes it difficult to impart a stable surface pressure to the molecules to form a monolayer. Accordingly, the most important factor for molecular planning of compounds for monolayer formation is to provide compounds which are well balanced in the hydrophilicity and hydrophobicity, which are insoluble in water and are nonvolatile.
As mentioned above, the molecular-oriented monolayer has particular hydrophilic groups or hydrophobic groups entirely on the surface of the layer and the groups are densely positioned on the plane face of the surface, and therefore the monolayer is characteristically and significantly different from any other cast film with no orientation. Because of the characteristics of such a monolayer, it is possible to build the monolayers one by one on a substrate having either a hydrophilic surface or a hydrophobic surface to form a multilayer film on the substrate, the thus formed multilayer film having particular hydrophilic groups or hydrophobic groups on the surface of the uppermost layer. Accordingly, when organic molecules having a hydrophilic or hydrophobic group with a particular functional groups are used, a functional organic ultrathin film can be obtained where the organic molecules are densely packed and oriented in the resulting multilayer with the functional groups facing towards the surface of the multilayer.
In general, the surface density of the molecules or functional groups in such a functional organic ultrathin film is from 10.sup.13 to 10.sup.15 molecules/cm.sup.2 or so, and the film has a characteristic feature that the thickness of the film can be freely controlled within the range of a minimum of 20 .ANG. or so.
An attempt has been made to apply a functional compound of a different kind (hereinafter referred to as a "guest compound") to the surface of such organic thin films or to insert a guest compound into the inside of the film, by virtue of a mutual reactivity of the guest compound and the hydrophilic group, hydrophobic group or functional group of the amphiphilic organic molecule of the film, so that the function of the guest compound is imparted to the thin film.
The most simple examples of such an attempt include: (1) a method where a guest compound is mixed with a monolayer-forming compound (hereinafter referred to as "host compound") in an appropriate developer solvent prior to film formation and the resulting mixture is spread over the surface of an aqueous subphase so as to form a mixed monolayer containing the host molecules, and (2) a method where a monolayer of a host compound is first coated on a substrate and then a monolayer of a guest compound is superposed thereover.
As one embodiment of such methods, for example, a study of a combination of a donor molecule and an acceptor molecule for control of the direction of transferring of electrons or energy has been reported in H. Kuhn, Proceeding of International Symposium on Future Electron Devices, page 1, 1985. In this case, however, since the guest compound is required to be a film-forming and hardly water-soluble compound like the host compound, the types of guest compounds that can be used are limited. In addition, when the guest compound is to be spread together with the host compound, a solvent which is compatible with both the host compound and the guest compound must necessarily be used, but the selection of such a solvent is not easy. Further, when the guest compounds are embedded into the monolayer of the host compounds to form a mixed monolayer, the resulting monolayer has a form where the guest compounds are inserted into the host molecules as a spacer, and therefore, there is a problem that the surface density of the guest molecules is limited to a low value.
As another embodiment, there is a method where guest molecules are applied to the monolayer of host molecules, which has previously been formed on the surface of an aqueous subphase or on the surface of a substrate, from a guest molecule-containing aqueous solution so that the guest molecules are attached to the surface of the monolayer or are inserted into the monolayer due to diffusion, adsorption or complex formation.
In this method, the adsorption of or complex formation with the guest molecules is generally accelerated by the hydrophobic bonding force or electrostatic attractive force between the molecules so that the adsorption and complex formation may become the equilibrated state. Utilizing this method, Fromherz et al. disclosed a process of applying guest molecules of bioactive proteins such as enzymes or the like to a host monolayer of fatty acids or esters by adsorption or insertion (FEBS Letters, Vol. 49, page 329, 1975).
In addition, also utilizing this method, another example where a monolayer to which a biological protein has been adsorbed is coated on a substrate by the L-B method is described in Japanese Patent Application (OPI) No. 251930/85 (FR-8407213) (the term "OPI" as used herein refers to a "published unexamined Japanese Pat. application"). However, since the monolayer formed by applying functional guest molecules to the layer by the adsorption method or complex formation method occurs via the chemical equilibrium of the adsorption or complex formation of the guest molecules, there is a problem in that the guest molecules are released from the monolayer by washing or similar mechanical action. Such a problem is a serious defect to practical use when a chemical reaction is to be conducted on the monolayer by way of the function of the guest molecules because the guest molecules peel away from the monolayer during the reaction and the function of the monolayer is lowered. In addition, in this embodiment, the functional groups of the guest molecules become oriented at random on the surface of the monolayer or they get embedded between the host molecules, and as a result, there is still another problem in that the function of the guest molecules can not be fully used.
Moreover, in accordance with this method, since the guest compounds indiscriminately adsorb to the surface of the thin film, it is impossible to control the part of the film to which the guest compounds are to be applied. This is a great problem in the case where the functional thin film is to be applied to a limited part of a small substrate such as a sensor or the like or in the case where the thin film is to be fixed as a pattern.